Do you have any photos you can share of the welded roof on the GA RR cars? I'd like to see them.
Disclaimer: Speculation Follows. I have a photo of the roof of a C&WC single-sheathed box car that was delivered about the same time as the GA RR cars in question. I was told by a trustworthy ACL historian that this was the same roof as the roof installed on the GA RR single-sheathed cars when delivered. The photos clearly show a flat panel, but the panels were riveted. Perhaps the roof panels were originally riveted and later welded. I can send those photos offline.
I'd be real interested to see your photos to see if they provide any additional information.
Thanks Bill and BTW the photos on the Dropbox site are amazing. You do great work!